Generating Ultrarelativistic Attosecond Electron Bunches with Laser Wakefield Accelerators

Abstract

In recent experiments ultra-relativistic femtosecond electron bunches were generated by a Laser Wakefield Accelerator (LWFA) in different regimes. Here we predict that even attosecond bunches can be generated by an LWFA due to the fast betatron phase mixing within a femtosecond electron bunch. The attosecond bunches are stable outside the LWFA and can propagate in vacuum many tens of centimeter without significant change in their duration. Our calculations show that evidence for the formation of attosecond bunches can be found in the spectrum of coherent betatron radiation from LWFA's. The generation of attosecond electron bunches would be of great interest to provide a new and unique tool for modern physical research. The potential of such bunches ranges from electron microscopy with attosecond resolution, to the generation of attosecond X-ray beams, for investigating physical, chemical and biological processes on the attosecond timescale.The shortest bunches to date are available from Laser Wakefield Accelerators (LWFA's) [1]. In recent experiments exploiting different parameter regimes [2,3,4,5,6,7,8,9], ultra-relativistic femtosecond (fs) electron bunches were generated by an LWFA. Here we predict that even attosecond bunches can be generated by an LWFA due to the fast betatron phase mixing withing a femtosecond electron bunch. The attosecond bunches are stable outside the LWFA, which means that they can propagate through vacuum over distances of many tens of centimeters, without a significant change of duration. We also predict how such bunches can be identified in the optical spectrum emitted during the acceleration process.Several schemes for the generation of attosecond electron bunches have been proposed so far. Examples are an inverse free-electron-laser process [10], the interaction of high intensity laser pulses with overdense plasma [11], the acceleration of electrons with a short tailored laser pulse [12], the slicing of an electron bunch with a laser pulse [13], the interaction of an ultra-short laser pulse with a nanofilm [14], with a wire or a plasma slice [15], or the interaction of a PW radially-polarized laser pulse with a sub-micron droplet of a high-Z material [16]. However, all these schemes are of limited attractivity, because they either require large accelerator structures [10] or high intensities in the order of 10 20 − 10 22 W cm −2 (normalized amplitude, a 0 , 10-100) [11,12,13,14,15,16], such as available only from rather exclusive (Petawatt) laser systems. A strong disadvantage is that these schemes would deliver bunches of rather low energy (a few to a few tens of MeV's) which would make it difficult to keep the duration of such bunches and apply them due to space charge effects. In some schemes the bunches would suffer from a limited life time (about 10 fs [14]) or a limitation of charge to well below a pC [10,12,16].Our scheme of attosecond bunch generation, presented in this Letter, has the advantage that it is based on the nowadays well-known technique of LWFA, and that the at...

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